Magnetic pole position estimating apparatuses for estimating the position of the magnetic poles in a salient electric motor have conventionally been known. A known example of such magnetic pole position estimating apparatuses estimates a magnetic pole position in an electric motor by superimposing a high frequency component on a pulse-width modulated (PWM) signal.
The magnetic pole position estimating apparatus disclosed in Japanese Patent No. 4670044, for example, estimates a magnetic pole position in an electric motor by generating a PWM signal to be applied to the respective three-phase windings in the following fashion. Considering successive three cycles of a carrier as one interval, the magnetic pole position estimating apparatus multiplies each original reference value by three for one third of the period (the first one cycle of the carrier), and superposes a high-frequency component for the remaining two third of the period (the remaining two cycles of the carrier).
The magnetic pole position estimating apparatus disclosed in Patent Literature 1, however, can update the original reference values for driving the electric motor only once in the three-cycle unit of the carrier.